An especially important requirement for turbomachines, especially gas turbines, which are used in power generating plants for electric power generation, is to ensure an aerodynamically stable operation of the turbomachine under largely all operating conditions which occur. In particular, a quick increasing of the ambient temperature, or a sudden drop of the network frequency of the network, must not be allowed to lead to aerodynamic instabilities of the flow of the respectively relevant compressor of the turbomachine, for example in the form of compressor surges.
Such critical operating conditions are customarily counteracted by means of control intervention by the load of the turbomachine being significantly reduced by means of a quick reduction of the fuel which is supplied.
However, in most cases this leads over a longer period of time to a reduced power output of the turbomachine until the turbomachine can be slowly run up again to its nominal power output. In many cases, the turbomachine, however, can also only be protected against greater damage, as can be caused by a compressor surging, by means of an emergency shutdown. This then means, however, that the turbomachine over a longer period of time completely fails and has to be first run-up again in a costly start-up process and synchronized with the network. A re-synchronization of the turbomachine with the network is also necessary when the turbomachine is decoupled from the system for a short time to avoid an aerodynamic instability.
This influence of ambient temperature and of network frequency which is proportional to the speed of the turbomachine, is reproduced in the parameter “aerospeed”:naero=nmech/Tambient0.5 
A reduction of the network frequency nmech, just as an increase of the ambient temperature Tamb, leads to a lower aerospeed naero. The lower the aerospeed, the lower is the capability of the compressor of the turbomachine to overcome the forming of aerodynamic instabilities. That is to say, the compressor with lower aerospeed naero has a smaller interval to the surge limit which limits the stable operating range of the compressor. This interval to the surge limit can be determined as “speed-surge margin”=SSM, which, as plotted in FIG. 2 in a compressor characteristic map 20 as a schematic illustration, is defined as the horizontal interval of the current operating point 24 from the point of intersection of the operating line 23 with the surge limit 22. The so-called “pressure-surge margin” PSM as the vertical interval of the current operating point 24 to the surge limit 22 represents a further stability parameter. This pressure-surge margin PSM, however, basically only plays a role when the compressor, with otherwise unchanged operation, has to deliver at a higher delivery pressure.
The problem of the risk of formation of aerodynamically unstable operating states, which is described above, occurs with increased effect in the case of “older” turbomachines, in which the compressor, as a consequence of operation, has recorded a power output deterioration. In addition to a power output deterioration, aging phenomena also lead to a lowering of the surge limit and consequently to a further reduction of the speed-surge margin SSM. In FIG. 3, in a further compressor characteristic map 20, the operating ranges for a new gas turbine and for a gas turbine which has already been in operation for a longer time, are exemplarily shown for this purpose. The compressor speed lines 21a-21g are shown as relative aerospeed lines in a range of from 90% to 105%, wherein 100% indicates the nominal operating speed at ISO ambient conditions. While the operating lines 23-1 and 23-2 of the two gas turbines (on account of unchanged throttle conditions) come to coincidently lie one above the other, the surge limit 22-2 which limits the stable operating range of the old compressor, compared with the surge limit 22-1 which applies to the new compressor, is appreciably shifted towards lower pressure ratios. Corresponding to the points of intersection 25-1 and 25-2 between the coinciding operating lines 23-1 and 23-2 and the respective surge limit 22-1 and 22-2, aerodynamic instability occurs at the aerospeed 21a (90%) in the case of the new compressor, whereas, however, aerodynamic instability already occurs at the aerospeed 21b (92.5%) in the case of the old compressor. Expressed in network frequency of the network and ambient temperature, this means that with a frequency drop of the network of 2.2 Hz, the new compressor would reach the surge limit at an ambient temperature of 50° C., whereas the old compressor would already reach the surge limit at 40° C.
Furthermore, it is also known that at low aerospeeds the aerodynamic instability is initiated in the front stages of the compressor. The stage loading is very high here at low speeds on account of the low mass throughput and the erroneous incident flow of the blades which is associated with it.
If a gas turbine is additionally operated with water injection or steam injection into the combustion chamber for increase of power output, then the compressor must deliver at a higher delivery pressure. This leads to a further increase of load of the compressor. As a result of this, both the speed-surge margin SSM and the pressure-surge margin PSM are reduced.
In addition, in the recent past power supply failures of greater extent were also recorded in addition to increasingly raised ambient temperatures, which led, and will further lead, to a further aggravation of the operating conditions for the turbomachines which are used for electric power generation. An aerodynamically stable operating mode of the turbomachines under all operating conditions in this case will increasingly become of crucial importance.